An extrusion molding is a molding method of pressing a work, such as a metal billet within a container and discharging an extrudate having a predetermined shape through an extrusion hole. Such an extrusion molding includes a direct extrusion method and an indirect method. The direct extrusion method is a method of manufacturing an extrudate by pushing one end of a work and discharging the extrudate through an extrusion hole disposed at a side opposite to the one end. On the contrary, the indirect extrusion method is a method of manufacturing an extrudate by discharging the extrudate in a direction opposite to a direction in which a work is pushed.
A metal pipe may be manufactured using the extrusion molding. When the direct extrusion method is used to manufacture a hollow pipe, a work passes through an extrusion hole comprised of a plurality of ports to obtain a plurality of metal pieces and then the plurality of metal pieces are again bonded in a solid state, thereby manufacturing a metal pipe. In this case, when the metal pieces that have separately passed through an extrusion hole in a solid state along a length direction of the metal pipe by the ports are again bonded to each other, a bonding trace is generated, which is called a seam line. For example, in the case of a molding having three extrusion holes each comprised of three ports, three seam lines are formed in an extrusion direction by a direct extrusion process. Since these seam lines are formed in an extrusion process by a mechanical bonding, although the bonding appears to be normal as viewed with naked eyes, such seam lines are actually mechanically weak portions, so when pressure of a fluid is generated in a metal pipe, the seam lines are easily damaged. In FIG. 11, a portion (marked with a circle) in which the seam line is burst is shown in a pipe expansion process in which hot-air forming of a metal pipe manufactured by the direct extrusion method is performed. It may be confirmed from FIG. 11 that when a high gas pressure is applied to an inside of the metal pipe, a weak seam line region does not withstand the high pressure and thus burst.
Since the indirect extrusion method does not go through a port during molding, a seamless pipe may be manufactured without a seam line that is a bonding of a solid state inevitably generated in an existing direct extrusion process. However, since the indirect extrusion method is performed by a batch method, it has disadvantages in that the process may not be continuously performed and a large force may not be applied compared to the direct extrusion method.
Various mechanical devices used in such an extrusion molding are installed in a narrow space, and thus are spatially limited. The mechanical devices include a driving source supplying power, and the driving source typically includes a motor operated by a magnetic force, a cylinder operated by a working fluid or the like. The more the driving source, the more difficult the installation due to a limitation of the space, so that the installation costs of devices increase, and an energy waste is generated in each of the driving sources, thus lowering work efficiency.